Welcome to our community

Be a part of something great, join today!

Log in
  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

"analog lenses" "digital lenses" backfocus

CinemaTechnic said:
That article got it wrong. You don't have to re-calibrate with each lens change. I discussed this issue with Alfred of P+S. But you might need more than one PL mount for the IMS, with each one optimized for a range of focal lengths. This is due to the digital cinema camera's focus sensitivity. Nothing to do with analog or digital lenses - there is no such thing.
Click to expand...

hola jorge-diaz,

thank you for the enlightenment.
i just was on point to believing it,
it makes sense that the light brakes in the front glass in the inner part. but a description that digital lenses have a more parallel optical path, hm, sounds wicked. where would than be the focal point with an more parallel optical path.

ok, i will keep my ears still open about this theme, perhaps somebody else is explaining something different.

thx y muchos saludos, manolo
 
Iron linear thermal expansion.
http://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion#Linear_thermal_expansion
http://en.wikipedia.org/wiki/Fe#Characteristics

Iron thermal expansion is 11.8 microns per meter per degree.
Well, with a bit of mathematics we see here that on 52mm (2”) distance at 40C deg temperature difference we will have 24.5 micro millimetre change.
So the change of length of 52mm long iron pipe is .61 micron per C degree

So without using special material for thermal compensation on the mount, Master Prime lens will go out of tune for infinity after 5C temperature change for 24mm wide 4K sensor and 10C change for 2K. However the focus ability is maintained up to 5-8 microns in the 3’-30’ without noticeable shift on the focus ring marking.

Other lenses could be more forgiving though.

I wonder, what is the temperature of the mount when RED is tuning the back focus in the factory?
Is camera just switched on or it is at the end of functional test where camera has already 90F body temperature. This will be the best temperature to choose for BF tune-up, I guess.

Finer, for Canada add 20 microns after tuning it up at room temperature when shooting infinity.
How many microns per one turn on RED? Anybody knows?
 
MANXLX said:
2)

all those lenses for analog fotocameras or cine-pl-lenses got a focus distance for filmemulsion. with digital cameras we got now a glass in front of the chip/sensor.
so what the articel says is, that with every different focal lenght lens, you would get different distances in backfocus (also if all lenses are really collimated correctly by 52,000mm).
this should happen by the glass in front of the sensor which breaks the light, so every lens with different focal lenght should get different angle to the chip.
the article said, that "digital lenses" like zeiss digiprimes(only 2/3 chip) and fotolenses for digital fotocameras got it solved, in manufacturing a more parallel light pass trough the last lens elements. so you don't have to adjust the backfocus by changing the lenses.
Click to expand...

i was thinking once again about the article in the german magazine "film und tv-kameramann" this would mean that all pl-lenses till today on market would be invalid for a red-one production,
or a godforsaken job for the AC to recollimate the backfocus after every lens-change,
or a own set of lenses with which each lens must be collimated professionally by a camerarental, and so each lens would get his own flange back (logical, this lenses then couldn't be used for a filmcamera, no kisses and no hugs for a kodak emulsion for this lenses)

..............puhhhhhhh:cold: thats sounds terrible, all three ways




i still don't like to believe that the glass in front of the sensor is an issue like the article described(and aslo like some people here in forum said). if it would so, than a zoom lens never could be right backfocused for the whole focaldistance.

please!!!! :help:
for sure somebody just made test with zoom lenses, not only optical, i mean also in measurement like AC's do.

report us the results of the test if you know 100% that your zoom lens is collimated by 52,000mm from an camera rental, and you know you set your backfocus right on the red-one.
tell us your zoom lens, and give us perhaps a photo report

thx

also let me know, if my thougt of this zoom test is not totaly correct for a verification
 
Focal Depth

Focal Depth

All cameras whether film or digital suffer from tempreture change. Plenty of 1st ACs carried PL or PV depth gauges to check in extremes of tempreture. Red is no different in this and the Master Primes may appear worse because of its maximum T stop where DOF is shallower and thus more prone to seeing the problem, if the camera is set-up correctly however in temporate climates with minimal termpreture changes it should be fine. However a chip is more prone than film.
So far no one has made digital lenses for 35mm they are only available for 2/3rds cameras where optical performance is more critical due to the smaller target size that said their is no short cut to optical performance and those that can only afford DSLR lenses are kidding themselves if they think consistantly they match professional cinematography lenses they dont.
 
PL Mount Back Focus / Thermal Expansion

PL Mount Back Focus / Thermal Expansion

Jim,

I did not mean to say that the RED PL mount drifts easily. It didn't in my experience. Once I got the right setting nailed down, I did not need to adjust it again. What I meant to convey, is my impression that you should simulate (or experience) the conditions you will be shooting in (temperature, etc.) when setting the back focus.

A also agree that lenses can and frequently will be miscalibrated when you get them. Sometimes they are set up on badly calibrated test equipment. I take great pains to make sure that my test equipment is carefully calibrated, and I have checked my gages against ARRI NY and have a NIST calibrated gage block set.

What I experienced was most likely due to not having calibrated the RED mount with the best accuracy in the first place. It was the first time I had done a calibration and I was not experienced doing it, nor did I have any set procedure to go on.

The first calibration was by eye (focus target and 17" LCD monitor via SDI), but in a well air conditioned studio (about 72º F). We were using a prime lens set that I had personally calibrated.

Once we started shooting, on a small, totally unventilated set, (probably about 104º F) and the camera warmed up (but did not overheat once), I noticed that the visual focus and measured focus did not match up. This is about a 30º temperature change from the temp the camera was set up at.

During I lunch break, I set up the still warm camera using the shortest lens in the set (Distagon 2/16mm) and a portable collimator. I carefully locked down the mount, before testing, checking with the collimator as I gradually tightened the cap screws. I also put a tape mark on the adjustment.

After making that adjustment, ALL the lenses were "dead nuts", focusing a Century Seimens Star at 6 ft with excellent accuracy (right on the witness line). I shot a quick test using the other side of the Century target and got a 56 lp/mm resolution in the final image. An impressive result, considering that this far exceeds the resolution of the RED LCD used to focus the camera (proving the accuracy of the calibration). After doing that, there were no focus issues for the next week and a half of the shoot.

I noticed that the thread on the PL mount adjustment was very fine. You could literally make micron adjustments.

As far as thermal expansion, I have experienced this with film cameras as well. It's just a matter of physics. Unless the lens mount is made from a 20 pound block of Invar, there is going to be some expansion.

Also people tend to forget - the lenses are thermally expanding too! You can't just blame the camera.
 
Thakns MANXLX,

On the feature I was working on, we were using Carl Zeiss T2.1 Standard Primes. Some of the optical designs are more than 30 years old (although with updated coatings).

ALL the lenses worked perfectly on the RED One and the images were beautiful. Exposure fall-off was not a significant problem. The RED One works just fine with film lenses.

There are film lenses that have very parallel light ray paths behind the rear element. This is not a "digital" lens, just an "Image Space Telecentric" lens. The Zeiss Distagon 2/10mm and 1.9/12mm lenses have telecentric exit pupils.

For me, one of the best features of the RED One is the huge choice of lenses that can be used with it. I always have said 3-CCD was a dead end where digital cinema was concerned.
 
thanks jorge for your report.

i hear so many things from all sides. perhaps some issues are bad lenses, some issues are like perhaps a bad manufactured camera(like the from hans von sonntag). the only way to know it better is to test each thing you will got in your hands.
the bad thing is that if someone gets an bad issue with something there beginns a rumor in da jungle.

greetz
 
You must log in or register to reply here.
Back
Top